Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 220802 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 402 AM EDT Sun Apr 22 2018 .SYNOPSIS...
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A large area of high pressure over eastern Canada and the Northeast United States will slowly build east and move off the New England coast on Monday. Meanwhile, low pressure over the western Gulf Coast states will track to the north and east and impact the Mid-Atlantic and Northeast for the middle of the new week. Another low may affect the area at the end of the week, and then a cold front may pass through the region next weekend.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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An upper-level jet streak will slowly move out of the northern Mid- Atlantic today, and this will allow for the slow downward trend in upper-level cloudiness during the day, even with a vort max progged to move through the region this afternoon. The region where this will take the longest to occur is the Delmarva Peninsula. However, I suspect this will matter little regarding maximum temperatures today, particularly as drier air advects into the upper levels this afternoon. GFS/NAM BUFKIT soundings suggest temperatures about 2-3 degrees warmer than yesterday (mixing to around 800 mb). With a surface ridge in place, winds should be light, which will inhibit stronger surface warming to some degree, so I suspect the warmer MAV MOS is probably a stretch. The past couple of weeks of verification indicate MAV guidance has a general 3-4 degree warm bias, whereas the MET guidance has very little bias (and generally the lowest errors among the statistical suite). Followed the MET closely today, and followed a mix of model- simulated 2-m temps/dew points for hourly values, which seem to mimic the diurnal evolution better. One thing to watch will be the development and progress inland of a sea breeze. According to high-resolution simulations, sea/bay breezes will likely progress well into New Jersey by late afternoon, and may reach Philadelphia by late in the day. With the sea/bay breeze passage, may see a burst of stronger winds and a slight drop in temperatures. Modified the grids to account for this, especially in portions of eastern/southern NJ and much of Delaware.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/...
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Main forecast concern tonight is the development of fog near the coast as remnant (light) onshore flow from the sea breeze combines with dropping temperatures to allow for low-level saturation. Spread the mention of fog a little bit further inland in southern/eastern NJ and coastal Delaware, though not expecting widespread/dense fog at this point. With the surface ridge in place, winds will likely decouple fast, and with clear skies thanks to being upstream of passing northwest- flow midlevel perturbations, temperatures will fall readily. I went a little below guidance for lows tonight, and would not be surprised to see the low-lying spots with a little frost late. However, suspect this will be quite patchy at best, so made no mention of this in the grids at this point. Forecast lows are similar to tonight`s values.
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&& .LONG TERM /MONDAY THROUGH SATURDAY/...
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Unsettled weather on tap for much of the new week as low pressure lifts along the coast. High pressure over the Northeast U.S. moves off the New England coast on Monday, and then slowly tracks out to sea Monday night and Tuesday. Onshore flow develops, and although temperatures climb into the 60s for most inland areas, temperatures will be colder along the coast with highs staying in the 50s. Low pressure tracking along the Gulf Coast states will be over the Southeast U.S. Monday evening, and then the low lifts along the coast Monday night and Tuesday. Onshore flow will user a increasingly moist airmass into the region, and PWATs will increase to around 1.25" Tuesday night and Wednesday morning. This may allow for a period of moderate to locally heavy rain during this time. The low will be off the Mid-Atlantic coast Wednesday morning, and then the low tracks north during the day Wednesday and will be north of the region by Wednesday night. From there, conditions generally dry out Wednesday night and Thursday, but there is the potential for unsettled weather to continue into Friday as several weak upper level disturbances pass through the region. There is the chance for another coastal low on Friday, as the CMC-GDPS indicates this low, but the GFS and the ECMWF do not. The GFS has a quick moving cold front that the ECMWF holds off until Saturday. For now, will carry chance PoPs Friday through Saturday due to low confidence. Temperatures during this time will generally be near or below normal levels for most of the week.
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&& .AVIATION /08Z SUNDAY THROUGH THURSDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...VFR with upper-level cloudiness gradually diminishing with time. Winds generally west or northwest around or below 10 kts. However, combination of sea/bay breezes may allow for a quick change to southerly winds around 10 kts during the afternoon (maybe with an occasional gust) at ILG, ACY, and MIV (maybe even PHL by late afternoon). Moderate confidence in sea breeze formation/timing; high confidence in VFR. Tonight...VFR with light/variable winds. High confidence. Outlook... Sunday night through Monday night...VFR. There is a small chance of visibility reduced in fog at KMIV and KACY on Monday morning. Winds will be light and variable for much of the period. High confidence, except the development of fog, for which confidence is low at this time. Tuesday through Thursday...MVFR/IFR conditions expected in rain and fog, especially Tuesday night through Wednesday afternoon. Easterly wind gusts up to 20 kt are possible Tuesday night into Wednesday. Otherwise, generally light winds (less than 10 kt) are expected. Medium confidence on flight categories, but low confidence on the timing of any flight category changes.
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&& .MARINE...
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Winds/seas will remain below advisory levels through tonight. Winds will become southerly during the afternoon, with speeds 10 to 15 kts, before becoming light and variable overnight. Seas will likely remain below 3 feet through the period. There is potential for at least patchy fog overnight, with visibility restrictions possible. Chances are particularly high in Delaware Bay and adjacent portions of the southern NJ/DE Atlantic coastal waters, where light easterly/southeasterly surface flow may remain for most of the night. Outlook... Sunday night through Monday night...Winds and seas are expected to remain below SCA criteria. Tuesday and Wednesday...East winds increase to 15-20 kt with gusts up to 25 kt, and seas build to 8-10 feet on the ocean. Wind gusts to 25 kt also expected on DE Bay. Wednesday night and Thursday...Winds shift to the northwest and subsequently decrease. The main question will be how long seas remain above 5 feet on the coastal waters. Current forecast is for elevated seas into late Thursday.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...MPS Near Term...CMS Short Term...CMS Long Term...MPS Aviation...CMS/MPS Marine...CMS/MPS

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